Consisting of the territory of the former Netherlands East Indies, Indonesia's main island groups are the Greater Sunda Islands, which include Java, Sumatra, central and S Borneo (Kalimantan), and Sulawesi; the Lesser Sunda Islands, consisting of Bali, Flores, Sumba, Lombok, and the western part of Timor; the Moluccas (Maluku), with Ambon, Seram, and Halmahera; and the Riau Archipelago. After years of dispute with the Dutch, W New Guinea (now Papua and West Papua) was formally annexed by Indonesia in Aug., 1969. The most important islands, culturally and economically, are Java, Bali, and Sumatra.
All the larger islands have a central volcanic mountainous area flanked by coastal plains; there are more than 100 active volcanoes. Earthquakes are frequent and, although not usually severe, can sometimes cause devastation. The islands of W Indonesia are subject to heavy rains during the rainy season (Dec.–Mar.), which often cause flooding and landslides. The animal life of Indonesia roughly forms a connecting link between the fauna of Asia and that of Australia. Elephants are found in Sumatra and Borneo, tigers as far south as Java and Bali, and marsupials in Timor and New Guinea. Crocodiles, snakes, and richly colored birds are everywhere. The tropical climate, abundant rainfall, and remarkably fertile volcanic soils permit a rich agricultural yield.
The population falls roughly into two groups, the Malayan and the Papuan, with many of the inhabitants east of Bali representing a transition between the two types. Within each group are numerous subdivisions, and cultural development ranges from the modern Javanese and Balinese to traditional tribes in Borneo, Sumatra, and New Guinea. The complex ethnic structure is the result of several great migrations many centuries ago, largely from Asia. The Chinese constitute by far the greatest majority of the nonindigenous population; they number about 2 to 3 million and play an important role in the country's economic life. There are smaller minorities of Arabs and South Asians.
More than 300 languages are spoken in Indonesia, but an official language, Bahasa Indonesia (a form of Malay), was adopted after independence and is now understood in all but the most remote villages. English is considered to be the country's second language, and Dutch is also spoken. Almost 90% of the population is Muslim, making Indonesia the largest Islamic nation in the world. Slightly less than 10% of the population is Christian, and about 2% is Hindu and 1% Buddhist. Hindus are concentrated principally on Bali, which is known for its unique culture. Animism, sometimes combined with Islam, is common among some groups.